Mail being delivered locally

We've got a problem with mail being delivered locally, we're using the DNS cluster setup and Server A (our main server) and server B (Reseller server) are both using this DNS cluster.

Server A tries to mail Server B however it's trying to deliver locally when it really should be delivering externally. This is common when servers accounts are both on the same server but these are seperate and the problem is new customers go onto Server B and can miss our account activation/support emails etc if they use an email account linked to the account (which they think will be fine as they've not updated dns yet).